High-prevalence of bovine leukemia virus in cow-calf operations: Implications for cow reproduction, culling, and calf weight.

Abstract

This study determined the prevalence of bovine leukemia virus (BLV) infection and proviral load (PVL) in cow-calf operations and evaluated their impact in productive metrics. A cohort of 1969 Angus-cross cows from 12 Virginia cow-calf operations was followed between Fall 2021 and Fall 2024. Blood samples were collected at two time points, one year apart, and tested by ELISA; PVL was quantified by qPCR in ELISA-positive cows. Pregnancy, culling, and weaning weight outcomes were analyzed using mixed logistic, Cox proportional hazards, and linear regression models. The PVL results were stratified into four categories according to quartile distribution. The overall cow-level prevalence of BLV was 76.0 % and 84.2 % at the first and second yearly sample, respectively, with herd-level prevalence averaging 78.8 % (SD = 14.96 %). For positive ELISA animals in the first bleeding, the average PVL value was 0.25 (SD = 0.34) For the second bleeding, the average PVL value for ELISA positive cows was 0.26 (SD= 0.34). No association was detected of ELISA-BLV status with pregnancy, culling, or calf weaning weight. However, cows in the high PVL category had lower pregnancy risk compared with those in the very low PVL category. Culling risk, survival and weaning weight were not associated with ELISA or PVL status. Proviral load values changed significantly from the first sampling to the second, with 72 % of the cows transitioning to lower PVL categories. These findings highlight dynamic within-cow PVL fluctuations and suggest an association between BLV PVL and reproductive performance. In contrast, BLV status was not associated with cow culling or calf weaning weight.